The invention relates to a method for estimating the interference level in a cellular radio system comprising in each cell at least one base station, and a number of subscriber terminals communicating with one or several base stations, in which method the terminals measure power levels from those frequencies which are on a list that is maintained by the terminals and comprise those base stations and frequencies which allow a handover to be performed by the terminal.
One of the most essential tasks in the radio system planning and maintenance is frequency planning, in which the resources available to a system are distributed to the different parts of the system. Frequency planning should be carried out with care, since it has a great influence on the capacity and the quality of the connections in the system.
In narrowband cellular radio systems, co-channel interference is one of the most significant factors restricting the system capacity. Co-channel interference means the interference of the signals transmitted at the same frequency in neighboring cells in a desired signal. Consequently, cellular radio systems have conventionally been designed to use the same frequency in cells which are at a sufficiently long distance from each other, whereby the interfering signals do not exceed the permissible limits due to the path attenuation of the signals. This results in a so-called frequency recurrence pattern in the cellular structure.
The estimation or measurement of interference levels is thus an essential part of frequency planning at each frequency. The frequency planning is performed in two steps: with the aid of computer simulation while the system is being set up and, on the other hand, with the aid of actual measurements when the system is already in use. Frequency planning is constantly needed since the systems develop and base stations are added or their locations are modified during the operating life of a system on account of increased traffic, for instance. When a system is in use, measurements have conventionally been performed by a separate measuring equipment arranged in a vehicle traveling in a target area.
A method in which the system itself provides for frequency planning is called Automatic Frequency Planning (AFP). The problem with AFP is gaining information about the actual interference conditions and the radio wave propagation on the frequency channels with no need to use costly, separate measuring arrangements and equipment.
The object of the present invention is to implement a method for gaining information required in frequency planning without separate measuring arrangements.
This is achieved by the method described in the introduction, the method being characterized in that the list further comprises frequencies whose power level information is used for the frequency planning of the system.
The method of the invention has several advantages. It is easy to implement since it does not necessitate separate investments in equipment. The desired frequency channels easily provide sufficient information for the needs of the frequency planning. The gathering and transmission of information occurs smoothly without disturbing the normal telephone traffic.